Consider the following subroutine in Fortran 77:

subroutine shift(a, b, c)

integer a, b, c

a = b

b = c

end

Suppose we want to call shift(x, y, 0) but we don’t want to change the value of y. Knowing that built-up expressions are passed as temporaries, we decide to call shift(x, y+0, 0). Our code works fine at first, but then (with some compilers) fails when we enable optimization. What is going on? What might we do instead?